A blade fuse is a protection element that interrupts an electric circuit promptly when an unexpected high current flows through it. Such blade fuses are now applicable to many fields.
As is known in the automobile field, for example, many fuses are used in a single automobile. The recent development of high-density mounting of electric circuit components has boosted a demand for the compactness of fuses to be mounted. In addition, an increasing number of fuses have been mounted.
However, on the contrary, a space allocated to a fuse box and the like has been increasingly narrowed. In such a case, when a normal current flows through a fuse box, many fuses therein emit heat from their fused sections, and this heat may shorten the lifetime. In addition, the heat is transmitted to an adjacent electric circuit through the terminal sections of the fuses, so that the electric circuit is heated over an extended period of time, which may cause the melting of the casing, the malfunction of the electric circuit, or eventually burnout of the circuit.
Accordingly, nowadays, the emergence of highly durable blade fuses in which a casing is not scorched within a normal, actually in-use current region is demanded. Those fuses have a fixed blown site, and their temperature does not increase greatly when currents flow through them.
There are some existing fuses adapted for the above application. A fuse of this type is interconnected at both terminal ends with a connection section, made of copper (melting point of 1050° C.) or a copper alloy, and its substantially central section is provided with a fused section (also referred to as a “narrow section”) having the smallest cross section. Furthermore, a low-melting-point metal piece, made of tin (melting point of 230° C.), silver, or the like, which is formed into a claw shape that rises above surrounding connection sections while surrounding the narrow section is swaged and fixed to an upper portion of the narrow section (e.g., Patent Documents 1 and 2).
The reason for fixing the low-melting-point metal piece to the narrow section is to promptly break and separate the narrow section as follows. When an overcurrent flows through the narrow section, the low-melting-point metal piece is melted. Then, the melted low-melting-point metal piece is diffused inside the base copper texture, creating a copper-tin alloy. In this alloy area, the melting point is lowered.
Unfortunately, if a metal bonding method by which a low-melting-point metal piece (made of zinc or zinc alloy) is fixed directly to an upper portion of a narrow section by means of, for example, swaging and is applied to a blade fuse for automobiles used in a relatively low rated current region in which a rated current is 30 A or below, a problem arises in that its rated current, fused site, and fused current cannot be controlled easily. The reason being is that the blade fuse is very sensitive to, for example, an oxide film formed between the metals or a trace quantity of dust, the rated current, fused location, and fused current becomes unstable.
Some fuses known in the art each include: a narrow section in which nothing is provided; and a rivet-shaped tin alloy having a low melting point which is fixed on both sides of the narrow section (e.g., Patent Document 3). These fuses are, however, intended for a high capacity field in which a rated current is 55 A. Furthermore, the length of the narrow section is 0.85 mm, but the distance between the narrow section and rivet-shaped tin is 3.81 mm. Thus, they are apart from each other by at least fourth times the length of the narrow section. This structure may prolong the time until the narrow section is blown and its temperature does not decrease easily when a current flows through it.
Patent Document 1: JP 2008-21488 A (claim 1, and a part indicated by reference sign 14 in FIG. 2)
Patent Document 2: JP 2745190 B1 (a part indicated by reference sign 110 in FIG. 8)
Patent Document 3: JP 7-31976 B (line 33 in column 10 to line 21 in column 11, and FIG. 5)